A tail unit fitted to ammunition generally comprises a plurality of fins uniformly distributed over the periphery to the rear of the ammunition. There are many reasons which may make it necessary, or at least preferable, to provide a tail unit having a span which is greater than the caliber of the ammunition. The presence of a tail unit is notably required when the ratio of the length to the caliber of the ammunition is high, the ammunition thus not being able to stabilize gyroscopically. A tail unit is also required for ammunition which is not stabilized gyroscopically and which is self-propelled in order to increase its range, as, for example, in the case of a rocket. A tail unit may also be used to optimize the accuracy of the trajectory of the ammunition. Such an optimization may notably prove indispensible for guided ammunition.
Generally, a tail unit is designed so as to adopt a folded position at rest, the tail unit fitting the caliber of the ammunition. This folded position makes it possible to reduce the spatial requirement of the ammunition and to facilitate the handling and storage thereof. For ammunition intended to be fired by means of a weapon, for example a launch rocket, the ability of the tail unit to adopt a folded position becomes imperative, the ammunition having to be introduced into a tube of which the diameter is adapted to the caliber of the ammunition. Such a tail unit is known as an unfolding tail unit. When the fins are folded in the longitudinal direction of the ammunition, the unfolding tail unit is said to be of the umbrella type.
The tail unit is open during flight, for example from the exit of the ammunition from the firing tube when the tail unit itself provides the stability of the ammunition or in the final approach phase in the case of guided ammunition. For an unfolding tail unit, the opening essentially consists of a rotation of the fins about their respective axis so that they protrude over the periphery of the ammunition. The rotation of the fins is also known as deployment. The deployment of a fin may be implemented by means of a control ring able to slide relative to the body of the tail unit and able to come to bear against one end of the fin. The control ring may be specific to each fin or common to all of the fins, which may make it possible to ensure the simultaneous opening of the fins. The forward movement of the control ring exerts a force onto the end of the fin and, by a lever arm mechanism, causes the rotation of the fin. The rotation of the fin is, for example, stopped by a bearing surface formed on the body of the tail unit. The control ring may comprise a planar part on which the end of the fin comes to rest at the end of travel of this control ring. The control ring and the fin axes form the principal means of the device for opening the tail unit. A locking device also has to be provided in order to reduce the rebound of the fins on the bearing surfaces to keep the fins in the deployed position and to avoid any return to the folded position.
Due to the relative movement of the different components of the tail unit, it is necessary to provide operating clearances. Said operating clearances have to be even greater if the control ring is common to all the fins. Moreover, other clearances are added to the operating clearances. Amongst these clearances are included clearances as a result of the range of shapes and sizes of the different components forming the tail unit and the clearances caused by the caulking of the bearing surfaces of the body of the tail unit when the fins come into abutment. Said last-mentioned clearances are generally significant, to the extent that the control ring moves forward very rapidly, causing the speed of deployment of the fins to increase. All these clearances are found in the open position, i.e. in the operational position when the fins are deployed. Typically, they introduce an angular clearance of each fin about its axis of greater than five degrees. As a result, said clearances impair the aerodynamic behavior of the ammunition. In particular, they impair the guiding of guided ammunition.